Antigen-specific cells can be expanded in vitro for long time periods (21 days). These cell populations, on subsequent challenge under conditions of limiting dilution, show very heterogeneous growth response characteristics depending upon challenge conditions chosen. For example, if TCGF is present with specific antigen, a tri-model LDA curve is found, suggesting interacting regulatory cells that are relatively infrequent and that can suppress the more common antigen-responsive cell population. Further, a more infrequent cell is present that is not subject to the regulatory cell. A similar behavior is found when mitogen plus TCGF is used to challenge the selected cell population, but only the frequencies of the relevant cell population are changed. This suggests that the selected population is subjected to general constraint of the sort mentioned above, regardless of stimulus. Cells stimulated with antigen alone display a simple behavior and the presence of TCGF seems to unmask the regulatory cell. In normal cell populations, unselected by prior antigenic challenge, exposure in vitro to Dextran Sulfate can also "unmask" the regulatory cell population.